For projection applications, laser sources in fundamental mode operation or monomode operation with ever higher powers are required in order, with increasing image diagonals of the projection area, to obtain sufficient brightnesses and nevertheless to achieve a very high efficiency. Moreover, in many cases a low aspect ratio of the laser beam is desirable in order to be able to simplify complex and lossy lens systems.
In typical index-guided laser structures such as, for instance, laser diodes having ridge waveguide structures, a monomode nature is achieved by the laser ridge having an extremely small width. However, this constitutes a significant disadvantage in the high technological requirements, since conventional exposure and etching technologies are encountering their limits. A further major disadvantage of such narrow ridge widths consists in the increased operating voltages necessary for operating such laser diodes.
A further possibility for improving the monomode nature of a laser diode having a ridge waveguide structure consists in a small ridge height. The resultant weak optical guidance has the effect that only the fundamental mode can establish oscillations. However, this simultaneously leads to increased threshold currents, since a weak wave guidance or a small height of the ridge is generally associated with current spreading.
It is furthermore known to apply absorber layers to a thin passivation layer alongside the laser ridge. However, since very thin passivation layers are required for this configuration, electrical problems can occur, for instance with regard to the breakdown strength or with regard to leakage currents. Furthermore, it is disadvantageous in this case that generally it is not possible to prevent the fundamental mode from also being damped by the absorber, which means losses in the laser parameters, in particular a reduced efficiency.